The present invention relates generally to feature measurement in scanning electron microscopy, and more specifically to apparatus and methods for enhancing image quality. The present invention may also be applied to feature measurement and image enhancement in similar instruments.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a sample 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the sample 104. The beam of electrons 102 either interact with the sample and cause an emission of secondary electrons 106 or bounce off the sample as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 10.
Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications. For example, on a sample made of a substantially insulative material (e.g., silicon dioxide), performing one or more scans over a small area sometimes causes the sample to accumulate excess positive charge in the small area relative to the rest of the sample. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area.
Thus, microscopy apparatus and techniques for improving image quality are needed. More specifically, mechanisms for controlling charge distribution on the surface of the sample are needed.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for controlling charge distribution on the sample so as to improve image quality. Charge is controlled by providing one or more electrodes over the surface to be scanned. Each electrode has an opening through which a beam of incident particles (e.g., electrons) may pass and strike the surface of the sample. A voltage (e.g., a negative voltage) is applied to the electrode such that at least some of the emitted particles (e.g., secondary electrons) from the surface are repelled away from the electrode and towards the surface to cancel positive charge that has built up on the surface. Additionally, voltage adjustments are made as a function of beam position so that charge is controlled and an image is generated during sample scanning.
In one embodiment, a method of controlling charge build up on a sample while an image is being generated is disclosed. The image is generated from a portion of the sample with a measurement device having a source unit for directing an electron beam substantially towards the sample. The measurement device also has a detector for detecting particles that are emitted from the sample, an electrode proximal to the sample having a hole through which the electron beam and a portion of the emitted particles may pass, and an image generator for generating the image of the sample from the detected particles. A first voltage is applied to the electrode when the electron beam is substantially in a center of the hole. The first voltage is selected to control positive charge build up on the sample. A second voltage is applied to the electrode when the electron beam is deflected a predetermined distance from the center of the hole. The second voltage is selected to allow a significant amount of emitted particles to reach the detector to facilitate image generation. In another embodiment, a continuous range of voltages is applied between the first and second voltage as the beam moves between the center of the hole and to the predetermined distance from the center.
In an alternative embodiment, the first and second voltages are selected such that a substantially same amount of emitted particles reach the detector when either the first voltage or the second voltage is applied. Preferably, the second voltage is also selected to control positive charge build up on the sample.
In another method aspect of the invention, the measurement device has an electrode proximal to the sample that is split into at least two portions forming a hole through which the electron beam and a portion of the emitted particles may pass. A first voltage is applied to the electrode portions when the electron beam is substantially in a center of the hole, and the first voltage is selected to control charge build up on the sample. A second voltage is applied to a first electrode portion when the electron beam is deflected a predetermined distance from the center of the hole while a third voltage is applied to a second electrode portion. The second voltage differs from the first voltage, and the second and third voltages are selected to allow a significant amount of emitted particles to reach the detector to facilitate image generation.
In an alternative embodiment, the electrode has a first and a second half. The first voltage is applied to the first and second halves when the electron beam is substantially in the center of the hole, and the third voltage is applied to the first half and the second voltage is applied to the second half when the electron beam is not substantially in the center of the hole. The third voltage has about a same value as the first voltage. In another embodiment, a continuous range of voltages is applied between the first voltage and the second voltage as the beam moves between the center of the hole and to the predetermined distance from the center.
In another embodiment, the invention pertains to an electron beam apparatus for generating an image from a sample. The apparatus includes an electron beam generator arranged to generate and control an electron beam that is directed substantially towards the sample, a detector arranged to detect charged particles emitted from the sample to allow generation of an image from the detected charged particles, and one or more electrodes arranged proximal to the sample such that a predetermined voltage may be applied to one or more electrodes such that charge build up on the sample is controlled while the electron beam is directed at the sample.
In another embodiment, the apparatus also includes a processor for controlling a plurality of voltages to the one or more electrodes as a function of the electron beam""s position. In yet another embodiment, the electrodes include a first and second electrode that each have bores through which the electron beam passes, and the first electrode are placed between the second electrode and the sample. The second electrode is a ground electrode and the first electrode is a wehnelt electrode.
In yet another embodiment, the electrodes include a first and second electrode that each have bores through which the electron beam passes. The first electrode is placed between the second electrode and the sample, and the second electrode is a ground electrode. Preferably, the first electrode is split into two halves such that a different or a same voltage may be applied to each half to control charge build up and to allow emitted particles to reach the detector.
The present invention has several associated advantages. For example, voltages may be adjusted on the described electrode arrangements such that positive charge build up on the sample is minimized. Additionally, the voltage adjustments may be synchronized with the raster scanning of the incident beam such that enough secondary electrons escape the sample to the detector. Thus, a clear image may be generated of the sample even when the incident beam is not within the center of the electrode hole or not normal to the sample.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.